Oxides of cobalt (Co), nickel (Ni), titanium (Ti), copper (Cu), molybdenum (Mo), lead (Pb), zinc (Zn), gold (Au), and silver (Ag) are important minerals. Various methods exist for recovering these compounds from the ores where they are found. For example, autoclave methods are often used to recover Co, Ni and Ti oxides. These methods are capital and labor intensive. Mo oxide has been leached by hydrochloric acid methods. Cyanide, thiosulfate, thiourea and halides are used in leaching Au and Ag metals and oxides. Cu. Zn and Pb can be leached with sulfuric acid.
Rutile (TiO2) is a mineral used for many purposes. Amongst other uses, it is a source of titanium metal and a paint pigment. Synthetic rutile is generally considered as any rutile created from another mineral, usually ilmenite, that has at least 90% TiO2. High purity rutile is 99.9%+TiO2. High purity rutile generally carries a commercial value premium.
Ilmenite (FeTiO3) is most often converted to synthetic rutile by. high temperature leaching with hydrochloric acid in an autoclave. Leaching temperatures are generally between 800 to 900xc2x0 C. Ferric chloride is sometimes used in these autoclave leaches to increase the reaction rates at the lower temperatures.
Zoumei Jin el al. (B. Mishra and G. J. Kiporous eds, In: Titanium Extraction and Processing, The Mineral, Metals and Materials Society (1997) pg 122-128) reported that 4 to 6 Normal hydrochloric acid at 110 to 140xc2x0 C., will dissolve the iron (Fe) from Ilmenite from the Sichuan province of China in 6 hours. They found that the reaction rate is 0.4 order with respect to initial Fexe2x88x922 concentration. They postulate a surface reaction control model with an apparent activation energy of 56.97 kilojoules per mol.
Conventional autoclave technology is capital, maintenance and energy intensive. The process disclosed in Zoumei Jin et al. process involves the use of large amounts of hydrochloric acid, which is expensive, difficult to handle and requires special stainless steel equipment. There is a clear need for more efficient processes for leaching of ores to obtain valuable minerals.
Cyanide is the most commonly used leachant for gold. Two molecules of cyanide complex with every molecule of gold. Copper also complexes with cyanide, but it takes 4 molecules of cyanide for every copper molecule. Copper is often present in copper gold ores in the one tenth to one percent range. Gold in these ores is in the one to 10 parts per million range. The copper consumes so much cyanide it needs to be recovered by hydrogen cyanide distillation, an expensive and dangerous operation. Systems have been proposed where sulfuric acid is used to leach the copper first. Then, the heap or batch is neutralized. Cyanide can then be used to leach the gold. The problem, of course, is the expense of neutralization. In heap operations, the additional worry of incomplete neutralization is present.
In other ores, the gangue, or unwanted material, can be an acid consumer. Copper oxide in limestone rich rock is an example.
The subject invention pertains to novel and highly efficient methods for leaching valuable minerals, such as cobalt (Co), nickel (Ni), titanium (Ti), copper (Cu), molybdenum (Mo), lead (Pb), zinc (Zn), gold (Au), and silver (Ag) from ores.
One aspect of the present invention concerns methods for recovering titanium from ores. One embodiment of the subject method uses an acidic solution, such as sulfuric acid, to leach titanium oxides from a mineral feed. Additional modifications and/or steps, including, for example, grinding of the ore, addition of an alkali metal halide, addition of a carbon source, and adjustment of pressure and/or temperature, can be incorporated in the process. In a preferred embodiment, a mineral feed is contacted with an acid and an alkali metal halide to leach titanium oxides from the feed. High purity titanium dioxide having a commercial premium over synthetic rutile can be produced using the methods of the subject invention.
Another aspect of the present invention concerns methods for recovering transition metals other than titanium from ores. In one embodiment, the present invention provides a method for recovery of nickel and cobalt from a mineral feed by leaching the feed with an acidic solution. In an exemplified embodiment, a mixture of sulfuric acid and an alkali metal halide are used to leach out cobalt and nickel from a laterite ore. The subject methods can also be used to recover cobalt, nickel, copper, etc. by leaching these elements from scrap metal.
The subject invention also concerns methods for recovering multiple metals or metal oxides in separate solutions. In one embodiment, ore is contacted with an acid solution, such as sulfuric acid. Solid residue is then collected and contacted with an alkali metal halide and acid solution. In an exemplified embodiment, the subject method is used to recover copper separately from gold and silver. The copper is recovered primarily in the first acid solution, while the gold and silver are recovered in the alkali metal halide and acid solution.